Cascaded condenser for multi-unit geothermal orc

ABSTRACT

A pair of organic rankine cycle systems are connected in series with the geothermal fluid passing first through an evaporator of the first system and then through an evaporator of the second system before returning to a sink. Similarly, the cooling tower is arranged to provide cooling water to pass first through the condenser in one system and then through the condenser of the other system, to reduce the total flow required and the size of associated cooling hardware.

TECHNICAL FIELD

This invention relates generally to vapor expansion systems and, more particularly, to additional efficiency improvements to a cascaded organic rankine cycle systems.

BACKGROUND OF THE DISCLOSURE

The well known closed rankine cycle comprises a boiler or evaporator for the evaporation of a motive fluid, a turbine fed with vapor from the boiler to drive the generator or the load, a condenser for condensing exhaust vapors from the turbine and a means such as a pump, for recycling the condensed fluid to the boiler.

Such rankine cycle systems are commonly used for the purpose of generating electrical power that is provided to a power distribution system, or grid, for residential and commercial use across the country. A source of heat to the boiler can be of any form such as fossil fuel, nuclear power, or waste heat from internal combustion engines, for example. Because of recent concerns for the environment, and because of the rapid rise in the price of petroleum products, the use of naturally occurring geothermal heat sources has become very attractive for use as the heat source for rankine cycle systems.

The organic rankine cycle (ORC) is a vapor power cycle with refrigerant (an organic fluid) instead of water/steam as the working fluid. A pump increases the pressure of condensed liquid refrigerant, and this liquid is vaporized in the evaporator/boiler by heat from the geothermal heat source, for example, and high pressure refrigerant vapor expands in the turbine, producing power. The low pressure vapor leaving the turbine is condensed before being sent back to the pump to restart the cycle.

In order to maximize the amount of energy that is derived from the geothermal heat source, two primary methods have been identified to maximize the energy extraction. One uses a plurality of such ORC systems in series with the condenser of the higher temperature system driving the evaporator of the low temperature system. Such a cascaded system is shown and described in pending U.S. application Ser. No. 11/886,281, assigned to the assignee of the present invention. While that application used a common heat exchanger for the condenser of the high temperature ORC and evaporator of the lower temperature ORC to maximize thermal efficiency of the generator, a second method uses a plurality of such ORC systems in series with the high temperature resource leaving the first evaporator partially cooled and entering the evaporator of a second system. This allows for minor hardware changes to maximize energy utilization such that the fluid exits the plants at the lowest possible temperature. This is accomplished by sequentially decreasing temperatures first through the higher temperature ORC design and then extracting additional energy when entering the final evaporator, thus providing greater resource utilization with common hardware design. This setup minimizes the parasitic pumping costs and maximizes the resource utilization associated with power production using the available heat in the resource flow. While that application used a common heat exchanger design for the condenser and evaporator of the higher pressure system and the lower pressure system, it was implemented with the condensers in parallel to maintain common condenser operating conditions.

The providing of cooling water to the condensers is commonly accomplished by way of a river, pond or a cooling tower. It is necessary to provide some motive power, such as gravity or by way of a pump, for moving the water that leaves the condenser back to the cooling sink or cooling tower. The parasitic power cost of this process is proportional to the amount of fluid used and the relation ship of cooling temperatures to the environmental conditions in the case of the cooling tower. Thus, pump and/or cooling fan power is required proportional to each of the cooling water systems, thus adding to the installation and operating costs It is desirable to reduce the installation and parasitic power costs.

DISCLOSURE

In accordance with one aspect of the disclosure, a pair of organic rankine cycle systems are arranged such that a source of cooling fluid is adapted to flow serially through a first condenser to cool the working fluid of a first system and then through a second condenser to cool the working fluid of a second system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a pair of cascaded organic rankine cycle systems in accordance with the prior art.

FIG. 2 is a schematic illustration of a cascaded pair of organic rankine cycle systems in accordance with the present invention.

DETAILED DESCRIPTION OF THE DISCLOSURE

Shown in FIG. 1 is a pair of organic rankine cycle systems 11 and 12 arranged to operate in accordance with the prior art. The first ORC system 11 includes, in serial, working fluid flow relationship, a pump 13, an evaporator 14, a turbine 17, and a condenser 19. The working fluid is any suitable organic refrigerant such as R-245fa. The refrigerant is pumped by the pump 13 to an evaporator 14 where it is heated by hot water from a geothermal heat source 16. The resulting superheated vapor then passes to the turbine 17 for driving a generator 18 to produce electrical power. The resulting lower energy vapor then passes to the condenser 19 where it is condensed by giving up heat to the cooling water circulating through the condenser 19 from a heat sink 21. The condensate then passes to the pump 13 to complete the cycle. The heat sink 21 may be a cooling tower or a pond or river, to the condenser 19, and a pump 25 drives the cooling fluid to the heat sink 21.

The second ORC system 12 includes in serial, working fluid flow relationship, a pump 22, an evaporator 23, a turbine 24 and a condenser 26 that operates in substantially the same manner as the first system to drive a generator 27. However, rather than the evaporator 23 being connected directly to the geothermal heat source 16 as in the first system 11, it is connected in series with the evaporator 14 such that the hot water, after having passed through the evaporator 14, flows along line 28 to the evaporator 23 for the purpose of vaporizing the liquid refrigerant in the second system 12. After passing through the evaporator 23, the low energy water then passes along line 29 back to the geothermal heat source 16.

The condenser 26 is fluidly connected to a heat sink 31 such that the cooling water passes from the heat sink 31 to the condenser 26 with the resulting warmer temperature water then being circulated from the condenser 26 back to the heat sink 31 by way of a pump 32 the pump, which is typically driven by an electric motor.

It should be mentioned that the heat sinks 21 and 31 may be separate heat sinks as shown or they may be a single heat sink that is connected to the two condensers 19 and 26 in parallel. However, the pumps 25 and 32 are separate and distinct in either case, and electrical power is required to drive those pumps.

Referring to FIG. 2, the first and second ORC systems 11 and 12 are shown with the geothermal fluid passing through the evaporator 14, with the temperature of the geothermal fluid dropping from a relatively higher temperature such as 285° F. to an intermediary temperature such as 220° F. The intermediary temperature fluid 220° F. then passes through the evaporator 23, gives up heat to the evaporator 23 and then drops to a relatively lower temperature of 170° F., after which it passes back to a sink 33.

Unlike the system as described hereinabove, the cooling water for the condensers 19 and 26 passes from the cooling tower 34 at a temperature of relatively lower temperature such as 85° F. to the condenser 19 where it is heated to an intermediary temperature such as 100° F. The intermediary temperature fluid then passes through the condenser 26 where it is additionally heated to relatively higher temperature such as 115° F. and then is returned to the cooling tower 34. It will be seen that the same amount of fluid is used for both process and only a single pump 36 is required for the two systems 11 and 12. Since, pump sizing and pumping cost are both driven by total flow rate, installation costs, as well as operating costs are reduced over the system as described hereinabove. So, because of the higher overall temperature rise for the cooling water route (i.e. 30° F. rather than 15° F.), the required water flow is reduced. This allows for reduced cooling tower size and lower parasitic power usage.

It should also be recognized that the cooling water flow in the condenser 19 and 26 is in counterflow relationship with the fluid flow in each of the systems 11 and 12.

While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims. 

1. A cascaded arrangement of a pair of organic rankine cycle systems with each having in serial working fluid flow relationship a turbine, a condenser, a pump and an evaporator, comprising: a source of high temperature fluid adapted to flow serially through a first evaporator to heat the working fluid of a first system and then through a second evaporator to heat the working fluid of a second system; and a source of cooling fluid adapted to flow serially through the first condenser to cool the working fluid of a first system and then through a second condenser to cool the working fluid of a second system.
 2. A cascaded arrangement as set forth in claim 1 wherein said source of higher temperature fluid is a geothermal source.
 3. A cascaded arrangement as set forth in claim 1 and including a single pump for pumping said cooling fluid in a closed circuit.
 4. A cascaded arrangement as set forth in claim 1 and including a single cooling tower for providing cooling fluid to said first condenser and receiving cooling fluid from said second condenser.
 5. A cascade arrangement as set forth in claim 1 and including a single cooling fluid stream wherein the working fluid passes through the condensers in counterflow relationship with the working fluid flow. 